Not applicable.
1. Field of the Invention
The present invention generally relates to coatings of the type used to protect components exposed to high temperature environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a predominantly beta-phase NiAl (xcex2NiAl) overlay coating alloyed with hafnium for use as an environmental coating and as a bond coat for a thermal barrier coating.
2. Description of the Related Art
Components within the turbine, combustor and augmentor sections of gas turbine engines are susceptible to oxidation and hot corrosion attack, in addition to high temperatures that can decrease their mechanical properties. Consequently, these components are often protected by an environmental coating alone or in combination with an outer thermal barrier coating (TBC), which in the latter case is termed a TBC system. Ceramic materials such as zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or other oxides, are widely used as TBC materials.
Various metallic coating systems have been used as environmental coatings for gas turbine engine components, the most widely used being diffusion coatings such as diffusion aluminides and platinum aluminides (PtAl). Diffusion aluminide coatings are formed by reacting the surface of a component with an aluminum-containing vapor to deposit aluminum and form various aluminide intermetallics that are the products of aluminum and elements of the substrate material. Diffusion aluminide coatings formed in a nickel-base superalloy substrate contain such environmentally-resistant intermetallic phases as beta NiAl and gamma prime (xcex3xe2x80x2) Ni3Al. By incorporating platinum, the coating further includes PtAl intermetallic phases, usually PtAl and PtAl2, and platinum in solution in the NiAl intermetallic phases.
Another widely used coating system is an overlay coating known as MCrAlX, where M is iron, cobalt and/or nickel, and X is an active element such as yttrium or another rare earth or reactive element. MCrAlX overlay coatings are typically deposited by physical vapor deposition (PVD), such as electron beam PVD (EBPVD) or sputtering, or by plasma spraying. MCrAlX overlay coatings differ from diffusion aluminide coatings as a result of the elements transferred to the substrate surface and the processes by which they are deposited, which can result in only limited diffusion into the substrate. If deposited on a nickel-base superalloy substrate, an MCrAlX coating will comprise a metallic solid solution that contains both gamma prime and beta nickel aluminide phases.
Used in combination with TBC, a diffusion aluminide or MCrAlX overlay coating serves as a bond coat to adhere the TBC to the underlying substrate. The aluminum content of these bond coat materials provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) protects the bond coat from oxidation and hot corrosion, and chemically bonds the TBC to the bond coat.
More recently, overlay coatings (i.e., not a diffusion) of predominantly beta-phase nickel aluminide intermetallic have been proposed as environmental and bond coat materials. The NiAl beta phase exists for nickel-aluminum compositions of about 30 to about 60 atomic percent aluminum, the balance of the nickel-aluminum composition being nickel. Notable examples of beta-phase NiAl coating materials include commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., which discloses a NiAl overlay bond coat optionally containing one or more active elements, such as yttrium, cerium, zirconium or hafnium, and commonly-assigned U.S. Pat. No. 6,291,084 to Darolia et al., which discloses a NiAl overlay coating material containing chromium and zirconium. Commonly-assigned U.S. Pat. Nos. 6,153,313 and 6,255,001 to Rigney et al. and Darolia, respectively, also disclose beta-phase NiAl bond coat and environmental coating materials. The beta-phase NiAl alloy disclosed by Rigney et al. contains chromium, hafnium and/or titanium, and optionally tantalum, silicon, gallium, zirconium, calcium, iron and/or yttrium, while Darolia""s beta-phase NiAl alloy contains zirconium. The beta-phase NiAl alloys of Nagaraj, Darolia et al., Rigney et al., and Darolia have been shown to improve the adhesion of a ceramic TBC layer, thereby inhibiting spallation of the TBC and increasing the service life of the TBC system.
The role that hafnium plays in improving spallation resistance of a TBC was investigated by Nagaraj and Rigney et al. Nagaraj investigated several Ni-49.8Al-0.5Hf alloys, and concluded that improved spallation resistance could be obtained if one or more oxygen-active elements, such as yttrium, hafnium, cerium, zirconium, etc., were present at levels of up to 1.0 atomic percent. Rigney et al. investigated other NiAl alloys containing hafnium, such as Ni-49.5Al-0.5Hf, Ni-48.5Al-1.0Cr-0.5Hf, Ni-49.45Al-0.5Hf-0.05Ga, Ni-48.3Al-1.0Ti-0.5Hf-0.2Ga, Ni-47.15Al-2.0Cr-0.25Fe-0.5Hf-0.1Y, Ni-48.05Al-0.75Ti-0.5Cr-0.5Hf-0.2Ga, Ni-44.3Al-5.0Ti-0.5Hf-0.2Ga, and Ni-45.5Al-1.04Ti-0.14Hf. Based on these results using up to 0.5 atomic percent hafnium, it was projected that, when combined with chromium, titanium, etc., as much as 2.0 atomic percent hafnium would be beneficial to a beta-phase NiAl bond coat.
Even with the advancements discussed above, there remains a considerable and continuous effort to further increase the service life of TBC systems by improving the spallation resistance of the thermal insulating layer.
The present invention generally provides a protective overlay coating for articles used in hostile thermal environments, such as turbine, combustor and augmentor components of a gas turbine engine. The invention is particularly directed to a predominantly beta-phase NiAl overlay coating for use as a bond coat for a thermal barrier coating (TBC) deposited on the overlay coating. According to the invention, hafnium at levels significantly higher than previously expected can be incorporated into a NiAl-based coating composition, yielding a coating system capable of exhibiting improved spallation resistance as compared to prior bond coat materials, most notably platinum aluminide diffusion coatings. A preferred composition for the beta-phase NiAl overlay bond coat of this invention further contains about 2 up to about 15 atomic percent chromium. Other possible alloying elements of the bond coat include titanium, tantalum and zirconium.
As a beta-phase NiAl intermetallic, the overlay coating of this invention contains 30 to 60 atomic percent aluminum, preferably in the stoichiometric amount with nickel, so as to be predominantly of the beta-NiAl phase. With the addition of hafnium, the overlay coating also contains hafnium intermetallic phases, such as beta prime (xcex2xe2x80x2) (or Heusler) phases (Ni2AlHf), which are believed to precipitate strengthen the beta-NiAl phase. With the addition of chromium, the coating benefits from improved hot corrosion, solid solution strengthening by chromium, and precipitation strengthening from fine xcex1-Cr phases dispersed within the beta phase of the coating. When present together in a beta-phase NiAl overlay bond coat, these phases have been shown to improve the spallation resistance of a TBC deposited on the overlay bond coat. The beta-phase NiAl overlay coating of this invention is also believed to have application as an environmental coating.
Other objects and advantages of this invention will be better appreciated from the following detailed description.